Childhood Obesity, and its associated metabolic complications, is rapidly emerging as one of the greatest global challenges of the 21st century. This application builds on a long track record of interdisciplinary collaboration among investigators using state of the art insulin clamp, MRS/MRI and stable isotope methodologies for noninvasively assessing muscle glucose metabolism in children. The first cycle of this grant started in 1991, investigating the correlates of Insulin Resistance, the most common metabolic complication of childhood obesity. Using epidemiological and physiological approaches we found that insulin resistance in obese children and adolescents is the best predictor of cardiometabolic risk factors and glucose intolerance and is associated with biomarkers of inflammation. These studies revealed a critical link between insulin resistance and altered tissue lipid partitioning in obese adolescents. The overall and continuing goal of the studies proposed in this competing renewal grant are to elucidate the putative mechanisms of insulin resistance in obese adolescents and to address a novel hypothesis regarding the potential role of Mitochondrial Dysfunction and Impaired Adipogenesis in its pathogenesis. The specific aims are: 1) to assess whether decreased mitochondrial function, using 31P Magnetic Resonance Spectroscopy (MRS), may contribute to the increased intramyocellular (IMCL) lipid content in obese adolescents; 2) to determine whether the obese phenotype characterized by high proportion of visceral fat and a low abdominal superficial layer is associated with an increased proportion of small adipose cells and reduced expression of genes regulating adipocyte proliferation and; 3) to determine prospectively whether the obese adolescents with a high proportion of visceral and low subcutaneous fat depots will develop a worsening of glucose tolerance. These hypotheses-driven patient oriented research studies will be performed by a multidisciplinary team of investigators from several departments: Internal Medicine: (Dr Gerald Shulman), Diagnostic Radiology (Drs Constable and Rothman), NIDDK (Dr Sam Cushman), using an integrated team approach.